This is the claim of a new article by Song and Giovannucci (2016) in JAMA, but I am skeptical. Here is why. The authors compare cancer incidence and mortality between a low and high risk group. They defined a patient as low risk based on not smoking, no or moderate alcohol use, BMI between 18.5 and 27.5, and regular exercise. When comparing this low risk group against a high risk group, they find that the a population attributable risk (PAR) of 25%-33% within their data set. However, their sample is made up of health professionals from the Nurses’ Health Study (NHS) and Health Professionals Follow-up Study (HPFS). They also compare the low-risk rates to the overall US population and find that the PAR is 41%-63% for cancer incidence, and 59%-67% for mortality.

Based on these figures, the author claim:

In the 2 cohort studies of US white individuals, we found that overall, 20% to 40% of carcinoma cases and about half of carcinoma deaths can be potentially prevented through lifestyle modification. Not surprisingly, these figures increased to 40% to 70% when assessed with regard to the broader US population of whites, which has a much worse lifestyle pattern than our cohorts.

If we just got people to exercise, stop drinking and smoking, could we really reduce cancer incidence and mortality to such a degree?

Almost certainly not. First, this is a correlation study. Consider the case of individuals who have some ailments. They may be more likely to exercise less, have higher BMI and also develop cancer. Increasing the exercise levels of patients who are already ill is unlikely to happen and even if it does is unlikely to decrease cancer rates by half. Even for relatively healthier people, if other factors (e.g., genetics) independently cause both negative health behaviors and cancer incidence, then the correlation between health behaviors is spurious.

Second, the out-of-sample extrapolation is not appropriate. Basically, it compares low risk nurses against the general population. If the results were causal–which it is likely not–then the causal interpretation would be that improving health behaviors and becoming a nurse or health professional decreases cancer. Clearly, becoming a nurse likely has little impact on cancer incidence but being a person who chooses to become a health professional is likely correlated with cancer incidence.

That being said, it is likely that better diet and exercise and less smoking and drinking will reduce cancer rates somewhat. Claiming that the reduction in cancer rates is 50% or more, however, appears to be a gross overestimate.